The Sagnac effect, in its original definition, is known as the relative phase shift between two beams of light that have traveled an identical path in opposite direction in a rotating frame. Generally speaking, the Sagnac effect deals with light that propagates along a circular or closed-loop path in a rotating medium. Phase accumulation along the path that is co-directed with rotation differs from that associated with the counter-directed one.
The Sagnac effect (i.e., the phase, accumulated by a light signal that propagates along a slowly rotating circular path, depends linearly on the system's angular velocity Ω) has been studied quite extensively in the literature. The interest stems not only from the theoretical view-point, but also from the practical one. Highly sensitive rotation measurement devices can be designed using this effect [S. Ezekiel and H. J. Arditty, editors, Fiber-Optic Rotation Sensors, Springer Series In Optical Sciences, Springer-Verlag 1982; H. J. Arditty and H. C. Lefevre, “Sagnac E ect in Fiber Gyroscopes,” Optics Letters, 6(8), 401-403 (1981); H. C. Lefevre, “Fundamentals of the Interferometric Fiber-Optic Gyroscope,” Optical Review, 4(1A), 20-27 (1997)]. For instance, modern fiber-optic gyroscopes, known as Sagnac interferometers, and used for navigation are based on this effect. They allow highly accurate measurements of rotation rates.
Some devices utilizing the Sagnac effect are configured as a ring-like interferometer, also called a Sagnac interferometer. Here, a beam of light is split into two beams. The two beams are made to follow trajectories in opposite directions. On return to the point of entry, the light is allowed to exit the device in such a way that an interference pattern is obtained. The position of the interference fringes is dependent on the angular velocity of the setup in which the device is installed. Usually, several mirrors are employed, so that the light beams follow a triangular or square trajectory. Optical fiber can also be employed to guide the light. The ring interferometer is located on a platform that can rotate. When the platform is rotating, the lines of the interference pattern are displaced sideways as compared to the position of the interference pattern when the platform is not rotating. The amount of displacement is proportional to the angular velocity of the rotating platform. The axis of rotation does not have to be inside the enclosed area.
Various optical gyroscopes are described for example in the following patent publications:
U.S. Pat. No. 4,445,780 discloses a Sagnac gyroscope, for measuring rotation rates. The gyroscope has an optical coupler, adapted for being fabricated by integrated optical techniques, which is compact and provides for operation of the gyroscope at quadrature for small rotation rates. The optical coupler is a symmetrical, channel waveguide structure comprising a two-mode central waveguide branching into three one-mode input waveguides at one end and into two one-mode output waveguides at the other end. The output waveguides are optically coupled to the ends of a fiber-optic loop which provides a closed optical path in which the Sagnac phase shift is produced. The middle input waveguide is adapted to transmit an incident beam into the optical coupler while the outer input waveguides are adapted to transmit the output beams of the Sagnac gyroscope to a circuit for measuring and comparing the intensities of the beams in the outer waveguides so that the rotation rate may be determined.
U.S. Pat. No. 6,163,632 discloses an integrated optical circuit for use in a fiber optic gyroscope which senses rotation rates by determining a phase shift due to the Sagnac effect between light beams traveling around an optical fiber sensing loop in opposite directions. A circuit is provided on a silicon-on-insulator chip comprising a layer of silicon separated from a substrate by an insulating layer. This circuit comprises: rib waveguides formed in the silicon layer for receiving light from a light source and transmitting light to a light detector; fiber optic connectors in the form of grooves etched in the silicon layer for receiving the respective ends of the optical fiber sensing loop; rib waveguides formed in the silicon layer for transmitting light to and from said fiber optic connectors so as to direct light beams in opposite directions around the sensing loop and receive light beams returning therefrom; phase determining means integrated in silicon layer for determining a phase shift between the light beams returning from the sensing loop.
JP 1143914 discloses a gyroscope configured for detecting the phase difference generated between two light beams which are propagated in an optical waveguide on an optical path substrate in the mutually opposite direction and guided out of both ends, and finding a rotary angular velocity. In this configuration, light from a laser is split by a beam splitter and made incident on both ends of optical fibers through lenses. Then, those light beams are guided to the optical waveguide on a transparent plate type medium and projected from the opposite sides of the fibers. Those light beams are collimated by the lenses, put together by the splitter, and made incident on a photodetector, which detects variation in the light intensity. When this gyro rotates at some angular velocity, the light beams have the phase difference calculated from a prescribed equation through Sagnac effect. Then light intensity detected by the photodetector varies periodically every time the phase difference reaches 2π, so the angular velocity is found from the output of the detector.
U.S. Patent Publication 2004/0202222 discloses a solid-state laser gyro, that comprises a solid-state resonator block, in which an optical path followed by two counterrotating waves generated by an optical-gain laser medium is defined, and the gain medium is attached to the resonator and is made of a rare-earth-doped crystal.
U.S. Patent Publication 2004/0263856 discloses a photonic crystal interferometric optical gyroscope system including a light source for providing a primary beam of light, a photonic crystal (i.e. a photonic crystal fiber) sensing coil having a rotational axis, and a beam controlling device configured to split the primary beam into first and second counter-propagating beams in the photonic crystal sensing coil and configured to direct return of the counter-propagating beams wherein the power of the returning counter-propagating beams represents the phase shift between the counter-propagating beams and is indicative of the rate of rotation of the coil about the rotational axis.